Jia-Zhuang Xu

Graphene Oxide Nanosheet Induced Intrachain Conformational Ordering in a Semicrystalline Polymer

The physical origin of graphene oxide nanosheet (GONS)-driven polymer crystallization was studied from the perspective of intrachain conformational ordering. Time-resolved Fourier-transform infrared spectroscopy indicated that both conformational ordering and crystallization of isotactic polypropylene (iPP) were obviously accelerated by the presence of GONSs, indicating their efficient nucleation activity for iPP crystallization. Furthermore, the ordering of long helical segments occurred prior to the crystallization of iPP, as revealed by two-dimensional correlation infrared analysis. Compared to pure bulk system, the presence of GONSs was in favor of the formation of long ordering segments, especially at the early stage, accompanied by considerable enhancement of the crystallization kinetics. GONS-driven iPP crystallization was suggested to be attributed to this GONS-induced intrachain conformational ordering.

Tuning the Superstructure of Ultrahigh-Molecular-Weight Polyethylene/Low-Molecular-Weight Polyethylene Blend for Artificial Joint Application

An easy approach was reported to achieve high mechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE)-based polyethylene (PE) blend for artificial joint application without the sacrifice of the original excellent wear and fatigue behavior of UHMWPE. The PE blend with desirable fluidity was obtained by melt mixing UHMWPE and low molecular weight polyethylene (LMWPE), and then was processed by a modified injection molding technology-oscillatory shear injection molding (OSIM). Morphological observation of the OSIM PE blend showed LMWPE contained well-defined interlocking shish-kebab self-reinforced superstructure. Addition of a small amount of long chain polyethylene (2 wt %) to LMWPE greatly induced formation of rich shish-kebabs. The ultimate tensile strength considerably increased from 27.6 MPa for conventional compression molded UHMWPE up to 78.4 MPa for OSIM PE blend along the flow direction and up to 33.5 MPa in its transverse direction. The impact strength of OSIM PE blend was increased by 46% and 7% for OSIM PE blend in the direction parallel and vertical to the shear flow, respectively. Wear and fatigue resistance were comparable to conventional compression molded UHMWPE. The superb performance of the OSIM PE blend was originated from formation of rich interlocking shish-kebab superstructure while maintaining unique properties of UHMWPE. The present results suggested the OSIM PE blend has high potential for artificial joint application.

Role of ion-dipole interactions in nucleation of gamma poly(vinylidene fluoride) in the presence of graphene oxide during melt crystallization.

The crystallization behavior and crystalline structure of poly(vinylidene fluororide) (PVDF) in the presence of graphene oxide (GO) platelets were investigated using time-resolved Fourier transformation infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), as well as differential scanning calorimetry (DSC). It is shown that GO platelets induce the formation of γ phase when crystallizing from solution, but only α phase forms from melt crystallization. The crystallization kinetics of α phase is promoted due to heterogeneous nucleation ability of GO, which is probably originated from a weak π-dipole interaction between GO and PVDF. Intriguingly, after introduction of strong ion-dipole interactions between GO and PVDF by addition of an ionic surfactant (cetyltrimethylammonium bromide, CTAB), a significant amount of γ crystals are obtained during isothermal melt crystallization. Time-resolved FTIR results further provide a detailed evolution of the γ phase formation, and there are two distinct stages during the melt crystallization in the PVDF/GO composites in the presence of CTAB, i.e., a simultaneous growth of γ and α phases in the first stage, and a solid α to γ transition in the second stage. These results may provide a facile routine to manipulate the crystalline structure in PVDF/GO composites, and thus to gain desirable properties.

Mechanical properties and biocompatibility of melt processed, self-reinforced ultrahigh molecular weight polyethylene

The low efficiency of fabrication of ultrahigh molecular weight polyethylene (UHMWPE)-based artificial knee joint implants is a bottleneck problem because of its extremely high melt viscosity. We prepared melt processable UHMWPE (MP-UHMWPE) by addition of 9.8 wt% ultralow molecular weight polyethylene (ULMWPE) as a flow accelerator. More importantly, an intense shear flow was applied during injection molding of MP-UHMWPE, which on one hand, promoted the self-diffusion of UHMWPE chains, thus effectively reducing the structural defects; on the other hand, increased the overall crystallinity and induced the formation of self-reinforcing superstructure, i.e., interlocked shish-kebabs and oriented lamellae. Aside from the good biocompatibility, and the superior fatigue and wear resistance to the compression-molded UHMWPE, the injection-molded MP-UHMWPE exhibits a noteworthy enhancement in tensile properties and impact strength, where the yield strength increases to 46.3 ± 4.4 MPa with an increment of 128.0%, the ultimate tensile strength and Youngs modulus rise remarkably up to 65.5 ± 5.0 MPa and 1248.7 ± 45.3 MPa, respectively, and the impact strength reaches 90.6 kJ/m(2). These results suggested such melt processed and self-reinforced UHMWPE parts hold a great application promise for use of knee joint implants, particularly for younger and more active patients. Our work sets up a new method to fabricate high-performance UHMWPE implants by tailoring the superstructure during thermoplastic processing.

Poly(L-lactic acid) crystallization in a confined space containing graphene oxide nanosheets.

The semicrystalline polymer incorporated with nanofillers frequently exhibits complicated crystallization behavior, which is probably attributed to the nanofiller-constructed complex crystalline circumstance, especially a confined space. In the present work, in order to have a thorough understanding of biodegradable poly(L-lactic acid) (PLLA) crystallization behavior on the dependence of graphene oxide nanosheet (GONS) loadings, in particular the relatively high GONS loading, a set of GONS/PLLA nanocomposites with different GONS loadings ranging from 0 to 4.0 wt % were investigated in terms of isothermal crystallization behavior by differential scanning calorimetry and time-resolved Fourier-transform infrared spectroscopy techniques. The results indicated that GONSs not only served as heterogeneous nucleating agents for PLLA crystallization but also restricted the mobility and diffusion of PLLA chains. At low GONS concentrations of 0.25 and 0.5 wt %, GONSs acted as a temple for PLLA chains to land on due to extremely high specific surface area, thus promoting the conformational ordering and reducing the nucleating barrier. The nucleation effect of GONSs was dominant to achieve accelerated overall crystallization kinetics. As the GONS concentration rose up to 1.0 wt %, the GONS network was formed in the PLLA matrix, which was verified by solid-like rheological behavior at low frequencies in rheological measurement. The nanofiller network significantly constrained the mobility and diffusion of PLLA chains and offset the nucleation effect of GONSs, giving rise to a turning point in crystallization rate from promotion to restriction. Furthermore, a severely confined space was constructed by the more crowded and denser GONS networks at a higher GONS concentration of 4.0 wt %, compelling PLLA lamellae to grow in a two-dimensional mode. The unusual crystallization behavior of PLLA from promotion to restriction was also understood by the four-region model, in which the semiquantitative description of crystalline circumstance was provided. These results pave an effective way to further reveal the crystallization behavior of polymer at a relatively high nanofiller loading.

Improved performance balance of polyethylene by simultaneously forming oriented crystals and blending ultrahigh-molecular-weight polyethylene

Polyethylene as a versatile polymer is being increasingly used for parts whose surfaces are in contact with moving metallic components or solid particles. This needs polyethylene to be greatly improved in mechanical properties as well as wear resistance. To this end, in the current work, various contents of ultrahigh-molecular-weight polyethylene (UHMWPE) were added into high-density polyethylene (HDPE) for enhancement of wear resistance, while the oriented crystals, i.e., shish-kebabs, were induced by shear flow for mechanical reinforcement. With 30 wt% UHMWPE was added, highly improved performance balance was achieved. The tensile strength rose from 26.4 MPa for normal HDPE samples to 68.5 MPa for the modified HDPE blends. The same trend was observed for impact toughness, where the impact strength increased from 6.3 to 34.1 kJ m−2. Moreover, addition of UHMWPE could reduce the wear rate from 22.1 to 7.6 mg MC−1. A very interesting phenomenon was observed, in which the overall properties of the modified HDPE blends were constantly enhanced with the increase of UHMWPE content though UHMWPE itself does not have much better mechanical properties than the oriented HDPE. This was ascribed to the amplified shear effect as a result of UHMWPE addition. The exceptionally high melt viscosity of UHMWPE assumes a gel state even at high temperature, making it just deform and hardly flow under the shear field, which amplifies the flow velocity difference between UHMWPE phase and HDPE melt. The amplified shear effect resulted in more pronounced molecular orientation and thus formation of a higher content of shish-kebab microstructure. Our work indicated that the melt processing-structure control strategy can desirably manipulate polyethylene products with desired properties.

Self-reinforced polyethylene blend for artificial joint application

By means of purposeful material design and melt manipulation, we present a highly feasible approach to simultaneously improve the mechanical properties, fatigue and wear resistance of an ultrahigh molecular weight polyethylene (UHMWPE)-based self-reinforced polyethylene (PE) blend for artificial joint replacement. The fluidity of the PE blend was achieved by blending low molecular weight polyethylene (LMWPE) with radiation cross-linked UHMWPE. The use of the cross-linked UHMWPE restrained the molecular diffusion between the LMWPE and UHMWPE phases, and hence increased the content of UHMWPE up to 50 wt% under the premise of desirable fluidity for injection molding. The combination of the shear flow field and pre-additive precursors successfully induced numerous interlocking shish-kebab structures in the LMWPE phase. Mechanical reinforcement was thus attained, where the ultimate tensile strength was significantly improved from 27.6 MPa for the compression-molded UHMWPE to 81.2 MPa for the PE blend, and the impact strength was increased from 29.6 to 35.2 kJ m-2. The fatigue and wear resistance were far superior to those of the compression-molded UHMWPE. Compared to the results reported in our previous study (40 wt% UHMWPE), the increased UHMWPE content caused the LMWPE phase melt to flow faster, thus amplifying the shear rate in the interfacial region between the two phases and depressing the relaxation of oriented molecular chains. The crystalline orientation was preserved, especially in the inner layer, leading to further enhancement of the mechanical properties. These results suggest that such a self-reinforced PE blend is of benefit to lowering the risk of failure and prolonging the life span of the implant under adverse conditions.

Surgical treatment for large spontaneous basal ganglia hemorrhage: retrospective analysis of 253 cases

Abstract Objectives. Spontaneous intracerebral hemorrhage (ICH) is a challenge to both neurologists and neurosurgeons. We aim to summarize the surgical treatment of ICH based on retrospective analysis of our patients. Methods. Two hundred and fifty-three patients with spontaneous ICH from August 2008 to August 2011 were retrospectively analyzed. Clinical data, including preoperative ICH score, pre- and postoperative GCS score, hematoma volume, postoperative brain infarction, 30-day mortality, and GOS 3 months postictus, were collected. One hundred and fifty patients had their intracranial pressure (ICP) monitored, and data were recorded and analyzed. All patients underwent craniotomy and clot removal under general anesthesia. Outcome analysis was stratified using hematoma volume, ICH score, preoperative GCS score, and decompressive craniectomy (DC). Results. The mean hematoma volume was 70.8 mL, and 68 patients (26.9%) underwent DC. The mean postoperative ICP was 28.8 ± 6.7 mmHg for patients without DC, and only 17.5 ± 8.6 mmHg for patients with DC. Twenty-five patients (9.9%) died within 30 days of operation, and 88 patients (34.8%, GOS ≥ 4) had good outcome 3 months after surgery. ICH volume > 50 mL, preoperative GCS score ≤ 8, and ICH score ≥ 3 are risk factors for unfavorable outcomes. Conclusions. DC can be used for patients with low preoperative GCS score, and it effectively reduces ICP and 30-day mortality. Hematoma volume, preoperative GCS score, and ICH score are of predictive value for surgical outcome of large basal ganglia hemorrhage.

Nucleation Ability of Thermally Reduced Graphene Oxide for Polylactide: Role of Size and Structural Integrity

Following our previous work on graphene oxide-induced polylactide (PLA) crystallization [Macromolecules 2010, 43, 5000-5008], in the current work, we further revealed the role of size and structural integrity of thermally reduced graphene oxide (RGO) in PLA crystallization. RGO nanoplatelets with different architectures were obtained via bath and probe ultrasound (RGOw and RGOp). The average size of RGO decreased substantially with ultrasound intensity and time, where the generation of RGO edges constituted the translocation of functional group sites from in-plane to edges. The formation of sp(3)-configuration dominated in RGOw, whereas the partial recovery of sp(2)-configuration occurred in RGOp, giving rise to either the escalation of sp(3)/sp(2) ratio for RGOw or retrogradation of that for RGOp. Isothermal crystallization kinetics of PLA nanocomposites containing RGOw and RGOp was determined by in situ synchrotron wide-angle X-ray diffraction. The induction period and overall crystallization rate of PLA/RGOw nanocomposites were strengthened with diminishing platelet size because of more nucleation sites encouraged by redistribution of functional groups. However, the adverse situation was found in PLA/RGOp nanocomposites. The observed phenomenon was ascribed to the disruption of the internal structure, i.e., the C═C sp(2) π-bond network, which deteriorated the CH-π interaction between PLA and RGO. These results conclusively suggested that the size and structural integrity of RGO had a concerted effort to determine the final nucleation ability of RGO dispersed by ultrasound.

Suppressing of γ-Crystal Formation in Metallocene-Based Isotactic Polypropylene during Isothermal Crystallization under Shear Flow

The effect of shear flow on isothermal crystallization behavior of γ-crystals in metallocene-based isotactic polypropylene melt was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD). In the sample under weak shear (at strain of 300% for 30 s duration), simultaneous evolution of α- and γ-crystals occurred, and the final fraction of γ-crystals (fγ) was 0.66, which was identical to the undeformed sample (PP-Static). In this scenario, α-crystals probably served as effective seeds for nucleation of γ-crystals. In the samples under strong shear (at strain of 500% for 30 s duration or long-time continuous shear at strains of 100% and 500%), the sequential emergence of α- and γ-crystals was observed. In this case, molten polymer chains were probably constrained by the surrounding crystals after intense short-time shear and/or maintained their extended chain conformation after long-time shear. These oriented chains had little chance to form the γ-crystals directly, behaving very differently from the relaxed chains. Under strong shear fields, the emergence of γ-crystals was delayed or inhibited, whereas the fγ value was also decreased rapidly. A simple model for the possible pathway of γ-crystal formation in the strong shear environment was proposed.