Goy Teck Lim

A nanostructured carbon-reinforced polyisobutylene-based thermoplastic elastomer

This paper presents the synthesis and characterization of a polyisobutylene (PIB)-based nanostructured carbon-reinforced thermoplastic elastomer. This thermoplastic elastomer is based on a self-assembling block copolymer having a branched PIB core carrying -OH functional groups at each branch point, flanked by blocks of poly(isobutylene-co-para-methylstyrene). The block copolymer has thermolabile physical crosslinks and can be processed as a plastic, yet retains its rubbery properties at room temperature. The carbon-reinforced thermoplastic elastomer had more than twice the tensile strength of the neat polymer, exceeding the strength of medical grade silicone rubber, while remaining significantly softer. The carbon-reinforced thermoplastic elastomer displayed a high T(g) of 126 degrees C, rendering the material steam-sterilizable. The carbon also acted as a free radical trap, increasing the onset temperature of thermal decomposition in the neat polymer from 256.6 degrees C to 327.7 degrees C. The carbon-reinforced thermoplastic elastomer had the lowest water contact angle at 82 degrees and surface nano-topography. After 180 days of implantation into rabbit soft tissues, the carbon-reinforced thermoplastic elastomer had the thinnest tissue capsule around the microdumbbell specimens, with no eosinophiles present. The material also showed excellent integration into bones.

Highly Hydrophobic Electrospun Fiber Mats from Polyisobutylene-Based Thermoplastic Elastomers

This paper is the first report of electrospinning neat polyisobutylene-based thermoplastic elastomers. Two generations of these materials are investigated: a linear poly(styrene-b-isobutylene-b-styrene) (L_SIBS) triblock copolymer and a dendritic poly(isobutylene-b-p-methylstyrene) (D_IB-MS), also a candidate for biomedical applications. Cross-polarized optical microscopy shows birefringence, indicating orientation in the electrospun fibers, which undergo large elongation and shear during electrospinning. In contrast to the circular cross section of L_SIBS fibers, D_IB-MS yields dumbbell-shaped fiber cross sections for the combination of processing conditions, molecular weight, and architecture. Hydrophobic surfaces with a water contact angle as high as 146 ± 3° were obtained with D_IB-MS that had the noncircular fiber cross section and a hierarchical arrangement of nano- to micrometer-sized fibers in the mat. These highly water repellent fiber mats were found to serve as an excellent scaffold for bovine chondrocytes to produce cartilage tissue.

Numerical Simulation of Mechanical Properties of Cellular Materials Using Computed Tomography Analysis

The IUPAC study ‘‘Structure and Properties of Linear and Cross-Linked Structural Polyvinylchloride Foams’’ (IUPAC no. 2003-038-4-400) of the IUPAC Subcommittee ‘‘Structure and Properties of Commercial Polymers’’ elucidates the mechanical properties of Polyvinylchloride foams. In order to predict the mechanical properties of polymer foams several simplified models were developed which take into account the density and stiffness of the bulk material, but do not apply any morphological information. Previous results on Polyvinylchloride foams indicate that the cubic model of Gibson and Ashby is not suitable to predict their compressive modulus. In this article, we discuss several alternatives for the prediction of the mechanical properties. In particular, a novel approach is presented in order to transfer cell morphological data obtained by computer tomography to realistic finite element meshes for numerical simulations. This approach allows to take into account the statistical character of the foam morphology and can be applied to predict the mechanical properties of foams. First, a computer tomography analysis was performed to determine the size distribution of the polymer foams using a nondestructive technique. The computer tomography information was applied to build finite element meshes using a tessellation of modified Kelvin cell units or truncated octahedra of various cell sizes. Then finite element simulations were performed using these meshes in order to predict the compressive behavior of polymer foams. The results of the numerical simulations were in a good agreement with those of experimental data. In conclusion, the mechanical properties of cellular polymers can be adequately predicted by taking into account the inhomogeneous foam structure, in particular the cell size distribution.

New biomaterial as a promising alternative to silicone breast implants

One in eight American women develops breast cancer. Of the many patients requiring mastectomy yearly as a consequence, most elect some form of breast reconstruction. Since 2006, only silicone breast implants have been approved by the FDA for the public use. Unfortunately, over one-third of women with these implants experience complications as a result of tissue-material biocompatibility issues, which may include capsular contracture, calcification, hematoma, necrosis and implant rupture. Our group has been working on developing alternatives to silicone. Linear triblock poly(styrene-b-isobutylene-b-styrene) (SIBS) polymers are self-assembling nanostructured thermoplastic rubbers, already in clinical practice as drug eluting stent coatings. New generations with a branched (arborescent or dendritic) polyisobutylene core show promising potential as a biomaterial alternative to silicone rubber. The purpose of this pre-clinical research was to evaluate the material-tissue interactions of a new arborescent block copolymer (TPE1) in a rabbit implantation model compared to a linear SIBS (SIBSTAR 103T) and silicone rubber. This study is the first to compare the molecular weight and molecular weight distribution, tensile properties and histological evaluation of arborescent SIBS-type materials with silicone rubber before implantation and after explantation.

Understanding Melt Rheology and Foamability of Polypropylene-based TPO Blends

Rubber-toughened polypropylene (PP) is an important resin for many engineering applications. Through the structural foaming of materials, material cost-saving and lightweight structures can be achieved. In this study, physical blends of PP and poly(ethylene/octene) at various compositions were characterized for their melt strength and shear viscosity. The high pressure MuCell ® foaming process was used to obtain structural foams with average cell diameters of <50 μm and cell densities of approximately 8 million cells/cm3. This study presents key understandings between material rheology and its suitability for structural foaming that are in turn linked to blend composition and melt temperature.

Investigation of structure-property relationships of polyisobutylene-based biomaterials: Morphology, thermal, quasi-static tensile and long-term dynamic fatigue behavior.

This study examines the morphology, thermal, quasi-static and long-term dynamic creep properties of one linear and three arborescent polyisobutylene-based block copolymers (L_SIBS31, D_IBS16, D_IBS27 and D_IBS33). Silicone rubber, a common biopolymer, was considered as a benchmark material for comparison. A unique hysteretic testing methodology of Stepwise Increasing Load Test (SILT) and Single Load Test (SLT) was used in this study to evaluate the long-term dynamic fatigue performance of these materials. Our experimental findings revealed that the molecular weight of polyisobutylene (PIB) and polystyrene (PS) arms [M(n)(PIB(arm)) and M(n)(PS(arm))], respectively had a profound influence on the nano-scaled phase separation, quasi-static tensile, thermal transition, and dynamic creep resistance behaviors of these PIB-based block copolymers. However, silicone rubber outperformed the PIB-based block copolymers in terms of dynamic creep properties due to its chemically crosslinked structure. This indicates a need for a material strategy to improve the dynamic fatigue and creep of this class of biopolymers to be considered as alternative to silicone rubber for biomedical devices.

Synthesis of POSS-Functionalized Polyisobutylene via Direct Initiation.

POSS-functionalized polyisobutylenes (PIBs) were synthesized by carbocationic polymerization using an epoxy-POSS/TiCl(4) initiating system in hexane/methyl chloride (60:40 v/v) solvent mixture at -80 °C. (1) H NMR spectroscopy verified the incorporation of one epoxy-POSS per polymer chain. Light scattering and TEM analysis demonstrated the formation of 50-100 nm sized aggregates and micron-sized clusters.

Novel Polyisobutylene-Based Biocompatible TPE Nanocomposites

Abstract The tensile and thermal properties of linear poly(styrene-b-isobutylene-b-styrene) (L_SIBS) and styrenic copolymers with a dendritic polyisobutylene core (D_SIBS) filled with 10 – 30 wt% of organophilic montmorillonite nanoclays (Cloisite(®)-20A) via solution blending were investigated. D_SIBS polymers were successfully reinforced by the clays without additional compatibilizers to show increase in both modulus and ultimate tensile strength. The clay platelets were well dispersed in the polymer matrix as determined by transmission electron microscopy (TEM). However, L_SIBS composites displayed decreasing tensile strength with increasing clay loading. TEM found clay agglomerates in L_SIBS composites that can act as “hotspots” for premature failure of the material. D_SIBSs loaded with 60 phr (37.5 wt%) carbon black (N234) also showed significant reinforcement. Interestingly, a D_SIBS with 17 wt% hard phase content reinforced with 60 phr carbon black exhibited an increase in the glass transition temp...

The effect of carbon black reinforcement on the dynamic fatigue and creep of polyisobutylene-based biomaterials

This paper investigates the structure-property relationship of a new generation of poly(styrene-b-isobutylene-b-styrene) (SIBS) block copolymers with a branched (dendritic) polyisobutylene core with poly(isobutylene-b-para-methylstyrene) end blocks (D_IBS), and their carbon black (CB) composites. These materials display thermoplastic elastomeric (TPE) properties, and are promising new biomaterials. It is shown that CB reinforced the block copolymer TPEs, effectively delayed the oxidative thermal degradation of the D_IBS materials, and greatly improved their dynamic fatigue performance. Specifically, the dynamic creep of a CB composite was comparable to that of chemically crosslinked and silica-reinforced medical grade silicone rubber, used as a benchmark biomaterial.