Forensic Engineering of Advanced Polymeric Materials-Part VII: Degradation of Biopolymer Welded Joints.

Welding technology may be considered as a promising processing method for the formation of packaging products from biopolymers. However, the welding processes used can change the properties of the polymer materials, especially in the region of the weld. In this contribution, the impact of the welding process on the structure and properties of biopolymer welds and their ability to undergo hydrolytic degradation will be discussed. Samples for the study were made from polylactide (PLA) and poly(3-hydroxyalkanoate) (PHA) biopolymers which were welded using two methods: ultrasonic and heated tool welding. Differential scanning calorimetry (DSC) analysis showed slight changes in the thermal properties of the samples resulting from the processing and welding method used. The results of hydrolytic degradation indicated that welds of selected biopolymers started to degrade faster than unwelded parts of the samples. The structure of degradation products at the molecular level was confirmed using mass spectrometry. It was found that hydrolysis of the PLA and PHA welds occurs via the random ester bond cleavage and leads to the formation of PLA and PHA oligomers terminated by hydroxyl and carboxyl end groups, similarly to as previously observed for unwelded PLA and PHA-based materials.

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.

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.

Effect of nanoscale confinement on glass transition of polystyrene domains from self-assembly of block copolymers.

The understanding of size-dependent properties is key to the implementation of nanotechnology. One controversial and unresolved topic is the influence of characteristic size on the glass transition temperature (T(g)) for ultrathin films and other nanoscale geometries. We show that T(g) does depend on size for polystyrene spherical domains with diameters from 20 to 70 nm which are formed from phase separation of diblock copolymers containing a poly(styrene-co-butadiene) soft block and a polystyrene hard block. A comparison of our data with published results on other block copolymer systems indicates that the size dependence of T(g) is a consequence of diffuse interfaces and does not reflect an intrinsic size effect. This is supported by our measurements on 27 nm polystyrene domains in a styrene-isobutylene-styrene triblock copolymer which indicate only a small T(g) depression (3 K) compared to bulk behavior. We expect no effect of size on T(g) in the limit as the solubility parameters of the hard and soft blocks diverge from each other. This strongly segregated limiting behavior agrees with published data for dry and aqueous suspensions of small polystyrene spheres but is in sharp contrast to the strong influence of film thickness on T(g) noted in the literature for free standing ultrathin polystyrene films.