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Claims  |
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What is claimed is:
1. A tube of ultra-high-molecular-weight polyethylene semicrystalline
morphology useful for vascular prostheses and having a Young's modulus of
at least 3 GPa obtained by wrapping an ultra-high-molecular-weight
polyethylene pseudo-gel on a mandrel, extracting the solvent with more
volatile solvent and evaporating volatile solvent.
2. The tube of claim 1 in which the porosity is adjustable over the range
of 0.2% to 90%.
3. A method for making an ultra-high-molecular-weight polyethylene product
for use in vascular prostheses and the like, comprising,
dissolving under slow-stirring conditions ultra-high-molecular-weight
polyethylene in a solvent at an elevated temperature in the range of
140.degree.-170.degree. C.,
cooling the solution at a rate less than 20.degree. C. per minute to a
temperature at which polymer crystals grow into an intercrystalline
network that produces a pseudo-gel state, and
extracting the solvent to produce a semicrystalline porous morphology.
4. The method of claim 3 wherein the cooling step is done under
non-isothermal, quiescent conditions.
5. The method of claim 4 followed by drawing the semicrystalline morphology
resulting from the extracting step, at a draw ratio of at least 5 and at a
temperature in the range from ambient temperature to a temperature not
exceeding the melting point of said semicrystalline morphology.
6. The method of claim 4, comprising, between the cooling and extracting
steps, the step of compressing the gel.
7. The method of claim 3 wherein the cooling step is done under isothermal,
non-quiescent conditions.
8. The method of claim 3 wherein the cooling step is done under isothermal,
quiescent conditions.
9. A method of making an ultra-high-molecular-weight polyethylene product
for use in vascular prostheses and the like, comprising,
dissolving, under slow-stirring conditions of about 60-600 r.p.m.,
ultra-high-molecular-weight polyethylene in a non-volatile solvent at an
elevated temperature in the range of 140.degree.-170.degree. C.,
cooling the solution at a rate less than 20.degree. C. per minute to a
temperature in the vicinity of 120.degree. to 125.degree. C. at which
polymer crystals grow into an intercrystalline network that produces a
pseudo-gel state, and
extracting the non-volatile solvent to produce a semi-crystalline porous
morphology.
10. The method of claim 9 wherein the cooling step is done under
non-isothermal, quiescent conditions, at a cooling rate of about 5.degree.
to 20.degree. C. per minute, without stirring.
11. The method of claim 10 followed by drawing the resulting product at a
temerature no lower than ambient and no higher than the melting point of
the semicrystalline porous morphology and at a draw ratio of at least 5.
12. The method of claim 10, comprising, between the cooling and extracting
steps, compressing the gel between plates at 100.degree. to 160.degree. C.
13. The method of claim 9 wherein the cooling step is done under
isothermal, non-quiescent conditions at a cooling rate of about
0.1.degree. C. per minute, while stirring.
14. The method of claim 9 wherein the cooling step is done under
isothermal, quiescent conditions at a cooling rate of about 0.1.degree. C.
per minute, without stirring.
15. A method for making a tubular profile of ultra-high-molecular-weight
polyethylene with adjustable wall thickness, comprising the successive
steps of
wrapping an ultra-high-molecular-weight polyethylene pseudo-gel containing
nonvolatile solvent on a mandrel,
extracting the solvent from the pseudo-gel with a more volatile solvent,
and
evaporating the volatile solvent from the tubular profile.
16. The method of claim 15, comprising making an anisotropic
ultra-high-molecular-weight polyethylene tubular structure by following
the evaporating steps with the step of stretching the tube on the mandrel.
17. The method of claim 16 in which the temperature of stretching is from
ambient temperature to the melting point of the semicrystalline porous
morphology obtained after the evaporating step.
18. The method of claim 16 in which the draw ratio is at least 5.times..
19. A method for making a tubular profile of ultra-high-molecular-weight
polyethylene with adjustable wall thickness, comprising the steps of:
dissolving said ultra-high-molecular-weight polyethylene in non-volatile
solvent at an elevated temperature in the range of 140.degree.-170.degree.
C.,
cooling the solution at a rate of less than 20.degree. C. per minute to a
temperature at which polymer crystals grow into an intercrystalline
network that provides a pseudo-gel,
wrapping the gel around a mandrel,
extracting the non-volatile solvent from the gel with a volatile solvent,
and
evaporating the volatile solvent from the tubular profile.
20. The method of claim 19 followed by stretching the tube on the mandrel
at a temperature from ambient to 135.degree. C. and at a draw ratio of at
least 5.times..
21. A method for making an ultra-high-molecular-weight polymer product for
use in vascular prostheses and the like, comprising,
slowly dissolving ultra-high-molecular-weight polymer in a solvent at an
elevated temperature,
cooling the solution at a rate below 20.degree. C. per minute to a
temperature at which polymer crystals grow into an intercrystalline
network that produces a pseudo-gel state, and
extracting the solvent to produce a semi-crystalline porous morphology.
22. The method of claim 21 wherein the cooling step is done under
non-isothermal, quiescent conditions.
23. The method of claim 22 followed by drawing the semicrystalline
morphology resulting from the extracting step, at a draw ratio of at least
5 and at a temperature in the range from ambient temperature to a
temperature not exceeding the melting point of said semicrystalline
morphology.
24. The method of claim 22, comprising, between the cooling and extracting
steps, the step of compressing the gel.
25. The method of claim 21 wherein the cooling step is done under
isothermal, non-quiescent conditions.
26. The method of claim 21 wherein the cooling step is done under
isothermal, quiescent conditions.
27. A method for making a tubular profile of ultra-high-molecular-weight
polymer with adjustable wall thickness, comprising the successive steps of
wrapping an ultra-high-molecular-weight polymer pseudo-gel containing
nonvolatile solvent on a mandrel,
extracting the solvent from the pseudo-gel with a more volatile solvent,
and
evaporating the volatile solvent from the tubular profile.
28. The method of claim 27, comprising making an anisotropic
ultra-high-molecular-weight polyethylene tubular structure by following
the evaporating steps with the step of stretching the tube on the mandrel.
29. The method of claim 28 in which the temperature of stretching is from
ambient temperature to the melting point of the semicrystalline porous
morphology obtained after the evaporating step.
30. The method of claim 28 in which the draw ratio is at least 5.times..
31. A method for making a tubular profile of ultra-high-molecular-weight
polyethylene with adjustable wall thickness, comprising the steps of:
slowly dissolving said ultra-high-molecular-weight polyethylene in
non-volatile solvent at an elevated temperature, in the range of
140.degree.-170.degree. C.,
cooling the solution at a rate lower than 20.degree. C. per minute to
temperature at which polymer crystals grow into an intercrystalline
network that provides a pseudo-gel,
wrapping the gel around a mandrel,
extracting the non-volatile solvent from the gel with a volatile solvent,
and
evaporating the volatile solvent from the tubular profile.
32. A pseudo-gel comprising a suitable solvent in an amount of 99 to 90
percent by weight and an ultra-high-molecular-weight polyethylene in an
amount of 1 to 10 percent by weight, said polyethylene being a
semicrystalline network with adjustable crystalline morphology comprising
randomly dispersed and oriented chain-folded single crystals, stacks of
single crystals, spherulite crystals, and extended-chain shish-kebab-type
of fibrils with lengths up to a few millimeters and widths up to 20 .mu.m.
33. The semicrystalline ultra-high molecular-weight polyethylene obtained
by removal of said solvent from the pseudo-gel of claim 32.
34. A semicrystalline morphology of ultra-high-molecular-weight
polyethylene comprising randomly dispersed and oriented single crystals,
stacks of single crystals, spherulitic crystals, and shish-kebab-type of
fibrils with lengths up to a few millimeters and widths up to 20 .mu.m, a
melting point of 125.degree.-140.degree. C., measured at a heating rate of
5.degree. C./min., a crystallinity of about 70%, measured on the basis
that the heat of fusion of a perfect polyethylene crystal is 293 J/g., and
a porosity from about 50 to 90%, measured on the basis that the density of
polyethylene is 960 Kg/m.sup.3.
35. A pseudo-gel comprising a suitable solvent in an amount of 99 to 80
percent by weight and an ultra-high-molecular-weight polymer in an amount
of 1 to 20 percent by weight, said polymer being a semicrystalline network
with adjustable crystalline morphology comprising randomly dispersed and
oriented chain-folded single crystals, stacks of single crystals,
spherulite crystals, and extended-chain shish-kebab-type of fibrils with
lengths up to a few millimeters and widths up to 20 .mu.m.
36. The semicrystalline ultra-high molecular-weight polymer obtained by
removal of said solvent from the pseudo-gel of claim 35.
37. A semicrystalline morphology of ultra-high-molecular-weight polymer
comprising randomly dispersed and oriented single crystals, stacks of
single crystals, spherulitic crystals, and shish-kebab-type of fibrils
with lengths up to a few millimeters and widths up to 20 .mu.m., a melting
point of 125.degree.-270.degree. C., depending on the particular polymer,
measured at a heating rate of 5.degree. C./min., a crystallinity of about
50% to about 75%. |
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Claims |
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Description |
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This invention relates to a novel ultra-high-molecular-weight polyethylene
product, useful for vascular prosthesis devices and for both industrial
and biomedical uses calling for high-impact, low-friction, high
wear-resistance, high-porosity, and softness or for one or more of these
qualities. The invention also relates to such devices. It further relates
to a novel method for processing solution-grown ultra-high molecular
weight polyethylene crystalline morphologies which may form pseudo-gel
states when the polymer is dissolved in a suitable volatile or
non-volatile solvent at an elevated temperature and the solution is cooled
to or below the temperature at which the polymer crystals are grown.
Additionally it relates to methods involving further processing of the
pseudo-gel states and their products after solvent extraction into
profiles and shapes with physical and mechanical properties that may be
tailored to specific biomedical (e.g., vascular and orthopedic prosthesis
and sutures) and industrial applications.
BACKGROUND OF THE INVENTION
Qualities such as softness, porosity, and biocompatibility have long been
sought in the field of vascular prosthesis.
The development of vascular prostheses has been a subject of extensive work
over the last 25 years. Most synthetic vascular prostheses have been
products of the application of textile technology in this field and have
been woven or knitted tubular structures designed to resemble the softness
and flexibility of the natural blood vessels. The two major synthetic
polymers used as vascular prostheses have been Dacron polyester and
polytetrafluoroethylene (PTFE).
These woven or knitted tubular structures are porous and have been produced
with smooth or velour surfaces. The porosity, which has been claimed to
play a determinant role for the healing process of the arterial
prostheses, can be controlled to some extent by adjusting the thread size,
or the interstices size, and by the texturization or knit pattern of the
synthetic fabric. High degrees of porosity may lead to excessive blood
loss after implantation; the velour has been claimed to have the advantage
that it fills the interstices of the underlying fabric, thus reducing
implant bleeding without reducing the porosity of the implant.
The DeBakey Ultra-light-weight Knitted Prosthesis (USCI, Inc.), the Cooley
Graft and the Wesolowski Weavenit (Meadox, Inc.) and the Microknit
(Golaski Lab, Inc.) are "smooth wall" commercial Dacron arterial
prostheses of different geometric and compositional configurations; the
Sauvage Filamentous Velour Prosthesis (USCI, Inc.) is an example of a
velour Dacron vascular prosthesis material.
Woven PTFE prostheses have low porosity and have not been used as much as
the Dacron prostheses. Also, it has been suggested that their use should
be temporary, their permanent use being "dangerous". More recently,
"expanded" PTFE, called "Gore-tex" (W. L. Gore & Assoc., Inc.) has become
available. "Gore-tex" is a network of small nodules interconnected by thin
fibrils and with an adjustable porosity from 0 to 96%, and it has been
used with generally encouraging results. Studies with smooth vascular
prostheses of expanded PTFE (85% porosity) and with ultra-light weight
woven PTFE showed that the patency (i.e., openness or non-occlusion) of
the expanded PTFE prostheses was significantly longer (4.5-10 months) in
comparison to the woven PTFE prostheses, which occluded in 101 days. It
was also demonstrated that the porosity played a critical role in the
healing process.
A recent theoretical calculation of the porosity in woven fabrics shows
that although their porosity can be designed to vary over a wide range,
the existing woven vascular prostheses, e.g. Woven Cooley and Woven
DeBakey prostheses have had a low degree of porosity which might not
permit complete healing, particularly in small diameter, low-blood-flow
locations where the 5 year patency rate is less than 30%.
Ultra-high-molecular-weight polyethylene (UHMWPE) is another polymer which
has attracted the interest of many workers for the preparation of
artificial prostheses, particularly the construction of orthopedic joint
devices, because of its outstanding abrasion resistance and strength.
UHMWPE, in contrast to the conventional high-density polyethylenes having
average molecular weights up to approximately 400,000, has an extremely
high molecular weight, typically 2-8 million, and is intractable. The
polymer is supplied as fine powder and is processed into various profiles
using compression molding and ram extrusion processes. The intractability
of the polymer can be overcome by varying the degree of material cohesion
and its initial morphology, more specifically by the formation of gel
states and single-crystal mat morphologies and by heating the polymer melt
to high temperature ranges, under inert conditions, in which the viscosity
of the melt is reduced significantly for melt processing. The preparation
of UHMWPE gel states and single crystal mat morphologies have been pursued
predominantly for the development of ultra-high modulus and strength
fibers. Melt processing of UHMWPE at high temperatures under inert
conditions has been investigated for the development of melt-crystallized
morphologies with enhanced mechanical properties which may result from the
material cohesion which is achieved by processing under such conditions.
Although the prior art covers to a large extent the preparation of
superstrong UHMWPE fibrous morphologies by spinning processes which
involve a gel intermediate, the focus of the works has been mainly on the
development of filamentary products with high modulus and strength in one
direction. Also, there has been an expressed desire that such filamentary
products have reduced porosity, because porosity may have an adverse
effect on the effective transmission of load within the oriented
filamentary products. On the contrary, the development of products with
bulk properties or enhanced isotropic mechanical properties from gel-like
precursors has received no attention. Furthermore, little effort has been
devoted to determining the effect of the morphology of the gel-like
precursor on the physical properties and the deformability of the products
from gel-like precursors. These areas fall within the scope of my
invention and they have a potential impact on the production of biomedical
devices such as vascular and orthopedic prostheses and sutures and also on
the fabrication of profiles possessing the outstanding wear properties of
UHMWPE.
An example of a stretched UHMWPE fiber and a process for making it is U.S.
Pat. No. 4,413,110. There a slurry of polymer in paraffin oil is heated to
between 180.degree. and 250.degree. C., preferably 200.degree.-240.degree.
C. and is then cooled to a temperature between -40.degree. C. and
+40.degree. C., the paraffin oil being replaced by a more volatile solvent
at a temperature below 50.degree. C. and the cooling being rapid and done
in such a way as to produce a "gel fiber". This "gel fiber" is then
treated to evaporate the more volatile solvent and to stretch the
"xerogel" fiber, as it is called, at 120.degree. C. to 160.degree. C.,
preferably above 135.degree. C. The porosity of the resultant fiber is
stated to be "no more than about 10% (preferably no more than about 6% and
more preferably no more than about 3%)".
SUMMARY OF THE INVENTION
The present invention includes the formation and use of UHMWPE pseudo-gels.
The term gel in the prior art referred to a macroscopically coherent
structure, that (1) is spatially cross-linked, (2) comprises a major
amount of low molecular-weight liquid, and (3) exhibits elastic properties
not unlike those of solids.
On the contrary, in this specification and in the claims the terms
"pseudo-gel" and "gel-like" refer to a concentrated solution of organic
polymer which contains an entangled three-dimensional semicrystalline
network the morphology of which may vary with the conditions of
preparation or crystallization, for example, when the pseudo-gel of this
invention is prepared under isothermal and quiescent conditions, the
crystals in the pseudo-gel have predominantly a lamellar morphology, with
single crystals or spherulitic crystals. The number of single crystals
decreases, and the crystalline morphology becomes more complex as the
solution concentration increases. On the other hand, when the pseudo-gel
of this invention is prepared under non-isothermal and quiescent
conditions, a large fraction of extended chain crystals (shish-kebab
crystals) are also present. However, because the molecular entanglements
in this semicrystalline network are not permanent, such solutions undergo
flow when a shear stress, no matter how small, is applied. Therefore, such
pseudo-gels exhibit time-dependent elastic properties and are not true
gels, as the term is properly used, because they do not possess an
equilibrium shear modulus.
In contrast to the prior art, the present invention provides for the
preparation of an UHMWPE crystalline morphology with isotropic mechanical
properties from a pseudo-gel precursor. Different crystalline morphologies
are obtainable under different processing conditions. This crystalline
morphology has an enhanced porosity which cannot be exhibited by
melt-crystallized morphologies, and it is readily deformable in the solid
state into products with different properties in one or more directions.
This invention provides a method for the preparation of UHMWPE vascular
prostheses by processing the UHMWPE in the pseudo-gel state. The
pseudo-gel is prepared by dissolving a suitable concentration of the
polymer in a suitable solvent, preferably a non-volatile solvent, at an
elevated temperature well above the temperature at which the pseudo-gel
forms and then cooling to or below the temperature at which the polymer
crystals grow and the pseudo-gel forms. The pseudo-gel is subsequently
processed under compression or tension into products of different
profiles. Extraction of the solvent from the shaped pseudo-gel products
leads to semicrystalline porous morphologies with geometrical
configurations analogous to those of their pseudo-gel precursors. These
morphologies may be processed further, at a temperature below or close to
their melting point.
This invention provides also a method for the construction of vascular
prostheses by (a) compressing the pseudo-gels into thin films between hot
plates, e.g., at 100.degree.-170.degree. C., (b) wrapping the pseudo-gel
film around a rotating mandrel into a multilayer tubular structure, and
(c) extracting the non-volatile solvent from the tubular structure on the
mandrel, for example extracting first with a more volatile solvent and
then drying out the tubular structure by evaporating the more volatile
solvent.
This invention also provides for the preparation of anisotropic UHMWPE
morphologies with enhanced mechanical properties, obtained by solid-state
deforming, i.e., at a temperature near or below the melting point of
UHMWPE, the semicrystalline polymer after extraction of the solvent, by
extrusion, drawing, molding, and forging techniques.
This invention also provides for the preparation of an UHMWPE
semicrystalline material which has adjustable porosity from zero up to
more than 90%. This material can be made with sufficient body to maintain
its shape and cross-sectional area; it is uniformly expensive and readily
deformable to generate products with bulk properties, in contrast to
filamentary products.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow sheet of a process embodying the principles of the
invention.
FIG. 2 is a flow sheet of another process embodying the principles of the
invention.
FIG. 3 is a flow sheet of a third process embodying the principles of the
invention.
FIG. 4 is an optical photomicrograph at about 100.times. of a UHMWPE
pseudo-gel morphology prepared under non-isothermal and quiescent
conditions (NIQ). The morphology was viewed with cross-polarized light.
FIG. 5 is a reproduction of a scanning electron micrograph of a UHMWPE
semicrystalline morphology obtained from the pseudo-gels after solvent
extraction and drying, according to the invention, the pseudo-gel having
been prepared under isothermal and non-quiescent conditions (INQ).
FIG. 6 is a similar scanning electron micrograph reproducion of a greater
enlargement of the same kind of UHMWPE semicrystalline morphology of this
invention the pseudo-gel having been prepared under isothermal and
non-quiescent conditions (INQ).
FIG. 7 is an optical photomicrograph at about 100.times. of a UHMWPE
pseudo-gel morphology obtained under isothermal and non-quiescent
conditions (INQ). The morphology was viewed with cross-polarized light.
FIG. 8 is a similar scanning electron micrograph reproduction of a
semicrystalline UHMWPE morphology of the invention obtained from the
pseudo-gel after solvent extracting and drying, the pseudo-gel having been
prepared under isothermal and quiescent conditions (IQ).
FIG. 9 is an optical photomicrograph at about 100.times. of a UHMWPE
pseudo-gel morphology obtained under isothermal and quiescent conditions
(IQ). The morphology was viewed with cross-polarized light.
FIG. 10 is a graph plotting Young's modulus versus draw ratio for a UHMWPE
of this invention. The solid-line curve represents observed values and the
broken-line curve shows estimated maximum values for the UHMWPE of this
invention.
FIG. 11 is a similar graph for a conventional polyethylene, but not a
UHMWPE.
FIG. 12 is a graph showing a plot of the shear modulus of a true gel in
dynes per square centimeter against frequencies from 1/100 to 1000 per
second.
FIG. 13 is a graph of the behavior of a pseudo-gel of this invention on
four successive frequency sweeps, with shear modulus (log GP) plotted
against the logarithm of the frequency .omega..
FIG. 14 is a graph showing the response of a true gel to temperature,
plotting shear modulus (as log GP, in Newtons per square meter) versus
temperature T in degrees centigrade.
FIG. 15 is a graph showing the response of a pseudo-gel of this invention
to temperature, plotting the shear modulus (log GP) against the reciprocal
of the temperature T; during a heating and cooling cycle at a frequency
.omega.=100 per second.
SOME DIFFERENCES IN BEHAVIOR BETWEEN TRUE GELS AND THE PSEUDO-GELS OF THIS
INVENTION
The contrast between true gels and a pseudo-gel is shown in FIGS. 12-15.
FIG. 12 shows that for a true gel, the shear modulus, up to a point, is
independent of the frequency. In contrast, FIG. 13 shows that the
pseudo-gel of ultra high molecular weight polyethylene (UHMWPE) in
paraffin oil is shear sensitive; (a) the shear modulus is dependent on
frequency, (b) the shear modulus is dependent on shear history; after one
or more shearing cycles there is always a different shear modulus at any
particular frequency. Here there were four cycles reading them from the
top down, on a 4% UHMWPE gel-like system in paraffin oil at 25.degree. C.
FIG. 14 shows that the shear modulus of a true gel is independent of
temperature, at least within a range, while FIG. 15 shows the hysteresis
behavior of a 3% UHMWPE pseudo-gel of this invention during a heating
(solid dots) and cooling cycle (hollow dots) at frequency .omega.=100 per
second. The shear modulus is dependent on temperature, decreasing
significantly with temperature. Also, during a thermal cycle, heating then
cooling, the initial shear modulus cannot be reached again.
DESCRIPTION OF SOME PREFERRED EMBODIMENTS OF THE INVENTION
As shown in FIG. 1, an UHMWPE pseudo-gel according to this invention may be
prepared from a raw UHMWPE powder 20 by dissolving the UHMWPE in a solvent
at 21, preferably a non-volatile solvent such as paraffin oil, in a
temperature range from 140.degree.-170.degree. C., preferably in the high
end of the temperature range in order to disrupt the thermally persistent
extended chain morphology of the raw UHMWPE powder and decrease the number
of nucleation sites which allow for the preparation, upon cooling, of
large lamellar single crystals. These are known to have a more regular
chain folded crystalline morphology than melt-crystallized and
solution-grown crystal morphologies obtained by rapid cooling, and
therefore have fewer intercrystalline and intracrystalline tie molecules.
Such tie molecules affect the deformation behavior of the semicrystalline
morphologies that are obtained after extraction of the solvent from the
pseudo-gel.
An UHMWPE used in this work was a HiFax 1900 (Hercules, Inc.) with an
average molecular weight of 2-8.times.10.sup.6. The UHMWPE was added
slowly to the paraffin oil (MCB Reagents, EM Science, PX 0045-3) to
concentrations from 1 to 8% by weight. To avoid degradation of the polymer
at high temperatures, the polymer is preferably stabilized with
approximately 0.5 wt. % (based on the polymer) of BHT antioxidant (butyl
hydroxy toluene) and heated under inert conditions, e.g., in nitrogen gas.
The mixtures are stirred slowly, at 60 to 600 r.p.m., under constant
conditions at a temperature of 150.degree. C. Under these conditions, a
clear solution was obtained within a few hours where the solution
concentration was about 1-2%,--within one day for a 2-3% concentration, up
to two days for concentrations of about 5%, and up to three days for
concentrations of 7-8%. A solution 22 was obtained that appeared clear
until it was cooled in step 23 to a temperature of approximately
123.degree. C., where it became opaque as the pseudo-gel was formed.
The uniformity of a so-produced UHMWPE pseudo-gel 24 depends on the
conditions of preparation. When the crystals are grown in the concentrated
solution under non-isothermal and quiescent conditions, herein called NIQ,
the cooling rate is about 5.degree. to 20.degree. C. per minute without
stirring, and the UHMWPE pseudo-gel is non-uniform and is comprised of a
mixture of single crystals and a fibrillar network in which the fibrils
have a shish-kebab crystalline morphology; the shish kebab fibrils in the
UHMWPE pseudo-gel can be up to 2-3 mm. long and 20 .mu.m. wide. This
non-uniform structure is shown in FIG. 4.
The semicrystalline morphologies in FIGS. 5 and 6 were obtained from the
pseudo-gel shown in FIG. 7 which was prepared under isothermal and
non-quiescent conditions, herein called INQ. The solution was cooled to
about 120.degree. C., at a cooling rate of about 0.1.degree. C. per
minute, for three to five hours and was then kept at about 120.degree. C.
for one hour. During the entire cooling process, the solution was stirred.
These semicrystalline morphologies were prepared by solvent extraction of
the pseudo-gel followed by drying. FIG. 5 is a scanning electron
micrograph of such a semicrystalline morphology; the degree of
magnification is shown by the white bar which corresponds to 0.1 mm. in
the actual semicrystalline morphology. A further magnification is shown in
FIG. 6, when the white bar corresponds to 10 .mu.m. FIG. 7 is taken from
an optical photomicrograph of the pseudo-gel at about 100.times. and shows
a typical shish-kebab fibrillar structure surrounded by randomly oriented
single crystals and stacks of single crystals, as viewed with
cross-polarized light.
Thus, UHMWPE pseudo-gels prepared under either non-isothermal or
non-quiescent conditions exhibit remarkable continuity and resistance to
shear deformation.
The pseudo-gel shown in FIG. 9 was prepared under isothermal and quiescent
conditions, herein called IQ. The same cooling process is used as for the
INQ conditions, but without stirring during the cooling process. When the
pseudo-gelation process occurs under isothermal and quiescent conditions,
the pseudo-gel is more uniform, has a more turbid texture, and consists
mainly of stacks of single crystals and large spherulitic crystals (up to
200 .mu.m in diameter) with a significantly diminished fraction of shish
kebab fibrils, and resistance to shear deformation. FIG. 8 is a scanning
electron micrograph of the semicrystalline morphology obtained by solvent
extraction and drying of a pseudo-gel like that of FIG. 9.
The different crystalline morphologies of the UHMWPE pseudo-gels obtained
under different conditions of preparation can be ascertained also by the
thermal behavior of the semicrystalline UHMWPE products which are obtained
by extracting, in step 25, the paraffin oil from the pseudo-gel precursor
with a more volatile solvent such as hexane, and subsequently evaporating
out, step 26, the volatile solvent off in a drying process, leaving a
semicrystalline UHMWPE morphology 27. The results of the thermal analysis
are summarized in Table I.
TABLE I
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Thermal analysis data of semicrystalline
UHMWPE morphologies from gel-like precursors
of different concentrations and different
processing histories.
Gel concentration
Tm Crystallinity
(% W/W) (.degree.C.)
(%)
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2 137 72
3 136.5 76
4 136.4 72
5 137 75
5* 129.8 73
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*Pseudo-gel preparation under isothermal and quiescent conditions.
The first column of Table I indicates the percent concentration of the
gel-like precursor, the second column the melting temperature of the
semicrystalline morphologies after the evaporation of the volatile
solvent, and the third column the percent crystallinity of these
semicrystalline morphologies. The melting endotherms were obtained at
5.degree. C./min. under N.sub.2, and the degree of crystallinity was
calculated, assuming that the heat of crystallization of a perfect
polyethylene crystal is 293 J/g.
The thermal analysis results indicate that the melting temperature of the
semicrystalline structures is independent of the concentration of the
gel-like precursors which were obtained under non-isothermal non-quiescent
conditions; however, it is lower when the gel-like precursor was prepared
under isothermal and quiescent conditions. The higher melting endotherm of
the crystalline structures at approximately 137.degree. C. from
pseudo-gels which were prepared under non-isothermal or non-quiescent
conditions indicates that the crystals should have an extended chain
configuration and is in agreement with the optical observations (e.g.,
FIG. 4) that so-produced pseudo-gels are comprised to a large extent of a
shish kebab fibrillar network.
The lower melting endotherm of the crystalline structure at 129.9.degree.
C. from a gel-like precursor which was prepared under isothermal and
quiescent conditions (see FIGS. 8 and 9) indicates that the crystals in
this case have a chain-folded configuration, and, again, this is in
agreement with the observation that the isothermally produced pseudo-gels
under quiescent conditions are comprised of chain-folded crystals, like
the stacks of single crystals and spherulites shown in FIGS. 8 and 9.
Also, it is clear from the data in Table I that the percent crystallinity
of the semicrystalline structure is independent of the studied
concentration range of its gel-like precursors as well as its processing
history.
A semicrystalline UHMWPE structure which is prepared from a pseudo-gel
intermediate combines morphological features which cannot be exhibited by
the morphologies obtained either by compacting the as-received fine powder
stock or by melt crystallization. These morphological features arise from
(a) the controlled material cohesion which is incomplete in the compacted
powders and the melt-crystallized morphologies prepared by partial fusion,
(b) the diminished, vis a vis the excessive, "amount of physical
entanglements" in the melt-crystallized morphologies prepared by complete
melting, and (c) the great variation of the crystalline morphology, from a
chain-folded to an extended chain crystalline morphology, depending on the
processing conditions during the preparation of the pseudo-gel precursor.
The importance of the first two factors relates to the balanced
enhancement of the deformability of the semicrystalline structures from
gel-like precursors and their mechanical properties. The role of the third
factor, and particularly the generation of a fibrillar network, comprised
of extended-chain shish-kebab crystals, is important because such
structure has a larger amount of free volume and consequently a high
degree of porosity.
In comparison to the knitted and woven textures whose porosity can be
adjusted to some extent by the thread size, interstices size, and the
texturization of the synthetic fabric, the intrinsic fibrillar networks of
this invention have the advantage that their porosity can be adjusted by
thermal and mechanical means.
Thermal treatment may be brought about by heating the semicrystalline
structure from a gel-like precursor to a temperature close to or above the
melting point of the polymer and then cooling to ambient under modest
compression (.ltoreq.approximately 50 Atm). This treatment resulted in a
significant porosity reduction.
Similarly, when a semicrystalline structure from a gel-like precursor was
compressed at ambient temperature under 500 Atm, its porosity was reduced
as a result of the densification process that takes place during the
compression process.
The porosity of the semicrystalline structure from a gel-like precursor may
be controlled also by the evaporation rate of the volatile solvent during
the drying process as well as by solid-state deformation. The effects of
pressure, temperature, and solid-state deformation on the percent porosity
of UHMWPE semicrystalline morphologies from gel-like precursors of
different concentrations are summarized in Table II.
TABLE II
______________________________________
The effect of temperature, pressure and solid
state deformation on the percent porosity of
UHMWPE semicrystalline morphologies from
gel-like precursors of different concentrations.
UHMWPE Sample Porosity (%)
______________________________________
NIQ - pseudo-gel, 1% 90.5
NIQ - pseudo-gel, 2% 51
NIQ - pseudo-gel, 4% 63
SSD - pseudo-gel, 4%/DR = 4
49
NIQ - pseudo-gel, 5% 49
SSD - pseudo-gel, 5%/DR = 8
40
INQ - pseudo-gel, 5% 78
IQ - pseudo-gel, 5% 79.5
NIQ - pseudo-gel, 5%/compressed under 500 Atm
0.2
NIQ - pseudo-gel, 5%/heated to 140.degree. C./50 Atm
8.7
NIQ - pseudo-gel, 8% 49
melt crystallized 7.3
______________________________________
NIQ = nonisothermal and quiescent conditions
INQ = isothermal and nonquiescent conditions
IQ = isothermal and quiescent conditions.
SSD = solid state deformed
DR = draw ratio
The porosity was calculated from density determination, assuming that the
density of polyethylene is 960 kgm.sup.-3. The symbol NIQ indicates that
the gel-like precursor was prepared under non-isothermal and quiescent
conditions; INQ indicates that the gel-like precursor was prepared under
isothermal and non-quiescent conditions; and IQ indicates that the
gel-like precursor was prepared under isothermal and quiescent conditions.
SSD indicates a semicrystalline morphology from an NIQ-pseudo-gel which
was deformed in the solid state by tensile drawing at ambient temperature
and at the indicated Draw Ratio (DR) determined by the cross-sectional
areas before and after drawing.
The data in Table II indicates clearly that the porosity of the
semicrystalline UHMWPE morphologies from gel-like precursors depends on
the conditions of preparation and can be adjusted over a wide range from 0
to 90% by a suitable choice of temperature and pressure, and solid-state
deformation.
As shown in FIG. 2, UHMWPE pseudo-gel states containing preferably a
non-volatile solvent can be processed by molding or rolling under
compression in step 31 to obtain thin gel-like film 32, preferably in a
temperature range close to or above the temperature at which the
pseudo-gel is formed (approximately 123.degree. C.). Subsequently, the
thin pseudo-gel film 32 can be wrapped in step 33 with or without tension
on a mandrel to give a multilayer tubular structure, a proces which allows
also for the build up of the tubular wall thickness. Cohesion of the
successive pseudo-gel layers is achieved by a molecular reptation process
which may occur between adjacent layers and result in a tubular wall with
fraying resistance.
The volatile solvent such as n-hexane replaced the non-volatile solvent in
step 25 and is removed by evaporation in step 34, to give a moderately
shrunk, hoop stressed structure 35, in a drying step. Typically, the
evaporation of the volatile solvent from the pseudo-gel is accompanied by
considerable shrinkage, which when constrained has the advantage of
resulting in the development of hoop stresses which enhance (a) the
molecular chain orientation in the circumferential direction and
consequently the lateral strength of the tubular structures and (b) the
molecular chain interpenetration in the overlapping pseudo-gel layers,
thus resulting in a coherent tubular wall.
Further enhancement of the mechanical performance of the tubular structures
can be achieved by solid-state drawing in step 36 of the hoop-stressed
dried tube 35. The process may result in tubular products 37 having a
biaxially oriented fibrillar network structure with mechanical integrity
along and across the draw direction. This integrity results from the
superposition of the circumferential orientation, which is obtained by the
constrained shrinkage during the evaporation of the volatile solvent, and
the axial orientation during the solid-state deformation process on the
mandrel. The solid-state deformation was performed at ambient temperature,
and the draw ratio (DR) was calculated from the displacement of markers on
the tubular wall after drawing.
FIG. 10 shows in a solid line curve average experimental modulus values at
different draw ratios. The variation of experimentally determined initial
modulus with the draw ratio is non-linear for the solid-state-drawn UHMWPE
semicrystalline morphologies of this invention to a draw ratio of 8, in
contrast to the previously established linear relation for
melt-crystallized, high-density (not ultra-high molecular weight)
polyethylene resins shown in FIG. 11; the difference is presumably due to
the high porosity of the UHMWPE semicrystalline morphologies, in
comparison to the low-porosity, melt-crystallized, high-density
polyethylenes, which diminishes and allows for effective load transmission
at significantly higher draw-ratios. FIG. 10 shows in a broken-line curve
a graph of experimentally determined maximum initial modulus values from
my new material at different draw ratios.
FIG. 3 shows a process much like that of FIG. 2. Pieces of pseudo-gel 40
are compressed at step 41 to make a gel-like sheet 42. The sheet 42 is
wrapped around a mandrel 43 to produce a gel-like tube 44, which is then
extracted at 45 to give a dry tube 46. This may be further processed, as
at 47, as by drawing, or may be received from the mandrel 43 to give a
vascular product 48.
Alternative methods for the preparation of tubular products with a
biaxially oriented fibrillar network structure include processes such as
solid-state extrusion through inverted conical dies and blowing air
through the tubes. Both processes are also suitable for the preparation of
biaxially oriented UHMWPE films which can be obtained by splitting the
biaxially oriented tubes along their length. Also, some or most of the
conventional thermoplastic processes, such as rolling, extrusion, drawing,
compression molding, forging, and extrudo-rolling are amenable to
processing UHMWPE semicrystallin | | |
