Arthur Wilkinson

Influence of Magnetic Field Alignment of Cellulose Whiskers on the Mechanics of All-Cellulose Nanocomposites

Orientation of cellulose nanowhiskers (CNWs) derived from tunicates, in an all-cellulose nanocomposite, is achieved through the application of a magnetic field. CNWs are incorporated into a dissolved cellulose matrix system and during solvent casting of the nanocomposite a magnetic field is applied to induce their alignment. Unoriented CNW samples, without the presence of a magnetic field, are also produced. The CNWs are found to orient under the action of the magnetic field, leading to enhanced stiffness and strength of the composites, but not to the level that is theoretically predicted for a fully aligned system. Lowering the volume fraction of the CNWs is shown to allow them to orient more readily in the magnetic field, leading to larger relative increases in the mechanical properties. It is shown, using polarized light microscopy, that the all-cellulose composites have a domain structure, with some domains showing pronounced orientation of CNWs and others where no preferred orientation occurs. Raman spectroscopy is used to both follow the position of bands located at ~1095 and ~895 cm(-1) with deformation and also their intensity as a function rotation angle of the specimens. It is shown that these approaches give valuable independent information on the respective molecular deformation and orientation of the CNWs, and the molecules in the matrix phase, in oriented and nonoriented domains of all-cellulose composites. These data are then related to an increase in the level of molecular deformation in the axial direction, as revealed by the Raman technique. Little orientation of the matrix phase is observed under the action of the magnetic field indicating the dominance of the stiff CNWs in governing mechanical properties.

Melting, reaction and recrystallization in a reactive PC-PBT blend

Abstract The crystallization behaviour of PBT and a 50 50 polycarbonate (PC)-poly(butylene terephthalate) blend with added transesterification catalyst were studied using differential scanning calorimetry (d.s.c.) and synchrotron small angle X-ray scattering (SAXS)/wide angle X-ray scattering (WAXS)/d.s.c. PBT crystallization was inhibited in the blend by both the presence of PC and transesterification. Increasing transesterification resulted in a progressive reduction in the melting and crystallization temperatures and degree of crystallinity, with the development of a mixed-phase glass transition at around 90°C. Transesterification also induced a significant change in blend morphology, from a coarse (5–10 μm) bicontinuous structure, when uncatalysed, to a sub-micron bicontinuous structure at low degrees of reaction.

Influence of ozone on styrene–ethylene–butylene–styrene (SEBS) copolymer

Non-commercial and commercial SEBS copolymer materials have been subjected to severe ozone treatment for different periods of time and concentrations in an ozone cabinet in order to obtain a deeper understanding on the mechanism of ozone attack in this type of material. The polymer materials were subsequently analysed by FTIR (ATR method), fluorescence and phosphorescence spectroscopy. Hydroperoxide analysis and determination of gel content was also carried out. Original functionalities in the SEBS based on aliphatic vinyl and aromatic (styrene) structures were observed to decrease in intensity and these were consistent with the concurrent formation of ozonide groups. Immediate exposure of SEBS to ozone resulted in the rapid and consistent formation of a variety of carbonyl and unsaturated carbonyl products based on aliphatic esters, ketones, and lactones as well as aromatic carbonyl associated with the styrene phase. These were followed by a more gradual formation of ether, hydroxyl and terminal vinyl groups with time and concentration. Also, of interest was the evident formation of a strong enol tautomer of a β-diketone functionality. These functional group changes were specific and concentrated on the very surface layer of the SEBS only. Whist there was strong evidence for hydroxyl group formation hydroperoxide analysis showed minimal evidence for active peroxides although growth was consistent with ozone dosage for the less ozone resistant materials. No crosslinking was also found in this material. Early decreases in in-chain vinyl groups by FTIR analysis were also consistent with an observed decrease in fluorescence functionalities in the SEBS associated with primarily trans-stilbene groups whereas longer periods of exposure showed new fluorescence functionalities. Phosphorescence analysis showed the formation of acetophenone end groups on the styrene chains associated with chain scission within the aliphatic rubber-styrene interphase region. Commercially ozone resistant SEBS materials were found to contain lower levels of fluorescent trans-stilbenic chromophores indicating this to be the weak link at the interphase in non-commercial ozone susceptible samples. Mechanistic routes for these processes are proposed and discussed.

Deformation micromechanics of all-cellulose nanocomposites: Comparing matrix and reinforcing components

All-cellulose nanocomposites, comprising two different forms of cellulose nanowhiskers dispersed in two different matrix systems, are produced. Acid hydrolysis of both tunicate (T-CNWs) and cotton cellulose (CNWs) is carried out to produce the nanowhiskers. These nanowhiskers are then dispersed in a cellulose matrix material, produced using two dissolution methods; namely lithium chloride/N,N-dimethyl acetamide (LiCl/DMAc) and sodium hydroxide/urea (NaOH/urea). Crystallinity of both nanocomposite systems increases with the addition of nanowhiskers up to a volume fraction of 15 v/v%, after which a plateau is reached. Stress-transfer mechanisms, between the matrix and the nanowhiskers in both of these nanocomposites are reported. This is achieved by following both the mechanical deformation of the materials, and by following the molecular deformation of both the nanowhiskers and matrix phases using Raman spectroscopy. In order to carry out the latter of these analyses, two spectral peaks are used which correspond to different crystal allomorphs; cellulose-I for the nanowhiskers and cellulose-II for the matrix. It is shown that composites comprising a LiCl/DMAc based matrix perform better than NaOH/urea based systems, the T-CNWs provide better reinforcement than CNWs and that an optimum loading of nanowhiskers (at 15 v/v%) is required to obtain maximum tensile strength and modulus.

Structure development in multi-block copolymerisation: comparison of experiments with cell dynamics simulations

Abstract A cell dynamics simulation of phase separation in block copolymers is compared with experimental observations for two related systems, polyurethane (poly(ether-urea)) foam and poly(ether-isocyanurate). Time resolved SAXS measurements on both systems suggest a spinodal-like mechanism with kinetics following a time-dependent Ginzburg–Landau (TDGL) model. TEM micrographs from a range of sources show reactively processed multi-block copolymers to have a bicontinuous morphology, which is discussed as a non-equilibrium relic of the phase separation process. A TGDL based cell-dynamics model gives predictions of the morphology, which can be compared to TEM images and SAXS patterns. The model does not contain any reactive aspects but captures the morphology of the systems which both showed pinning of the micro-structure at early stages of microphase separation in contrast to the equilibrium structures formed by block copolymers.

The effects of transesterification on structure development in PC-PBT blends

SummaryA series of polycarbonate-poly(butylene terephthalate) (PC-PBT) blends has been formed via reactive melt-blending in a torque rheometer. The degree of transesterification between the two homopolymers was controlled by the incorporation of an alkyl titanium catalyst. Resultant materials were characterised using DSC, DMA, FTIR and SEM. As the degree of transesterification increased composition of the blends became increasingly complex, comprising mixtures of the homopolymers and various AB-type block copolymers of PC-PBT, with concomitant changes in their thermal behaviour. A corresponding transformation in materials morphology was observed: the relatively coarse structure characteristic of essentially two-phase blends, developed into a more refined structure exhibited by blends containing significant volumes of interphase material. This morphological change was due to the formation of increasing concentrations of random block copolyesters.

Controlled Transesterification and Its Effects on Structure Development in Polycarbonate–Poly(Butylene Terephthalate) Melt Blends

A series of 50:50 polycarbonate–poly(butylene terephthalate) (PC-PBT) blends were formed via reactive melt blending in a torque rheometer. A controlled degree of transesterification between the two homopolymers was initiated by the incorporation of an alkyl titanium catalyst during melt blending and finally quenched by the addition of a transesterification inhibitor. The resultant materials were characterized using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). As the degree of transesterification increased, the composition of the blends became increasingly complex, comprising mixtures of the homopolymers and various AB-type copolymers of PC and PBT, resulting in significant changes in their thermal behavior. A corresponding transformation in the morphology of the blends was observed due to the formation of increasing concentrations of copolyesters. Thus, the initial coarse (>5 μm) bicontinuous morphology developed into a more finely dispersed submicron-scale structure, exhibited by blends containing significant volumes of interphase material; finally, at high degrees of transesterification, a homogeneous, amorphous material was formed.

Effects of soft-segment prepolymer functionality on structure–property relations in RIM copolyurethanes

Segmented copolyurethanes comprising 40–60% by weight of polyurethane hard segments (HS) and polyether soft-segment (SS) with different functionalities (SS-fn), have been formed by reaction injection moulding (RIM). The HS were formed from 4,4′ diphenylmethane diisocyanate (MDI) reacted with ethane diol (ED). The three SS-prepolymers used were all hydroxyl-functionalised poly(oxypropylene-b-oxyethylene)s with different nominal functionalities (fn) of 2, 3 and 4 but with a constant molar mass per functional group of ∼2000 g mol−1. RIM materials were characterised using differential scanning calorimetry, dynamic mechanical thermal analysis, tensile stress–strain and single-edge notch fracture studies. Predictions using a statistical model of the RIM-copolymerisation showed that increasing SS-fn lead to more rapid development of copolymer molar mass with isocyanate conversion. Experimentally, the RIM-PU exhibited a wide range of mechanical behaviour resulting from differences in molecular and morphological structures. Increasing SS-fn produced materials with improved mould release behaviour and fracture resistance. However, increasing SS-fn also reduced the degree of phase separation developed in the copolyurethanes, resulting in increased modulus–temperature dependence and poorer tensile properties.

A synchrotron SAXS study of structure development in a copoly(isocyanurate-urea) formed by RIM

Abstract Structure development during reaction injection moulding of a copoly(isocyanurate-urea) was studied using time-resolved, synchrotron SAXS. During the rapid copolymerization of liquid reactants, incipient microphase separation was shown to occur at a critical conversion of isocyanate groups and to proceed via the kinetics associated with spinodal decomposition. Microphase separation was halted prematurely by vitrification of the polyisocyanurate phase, thus producing a copoly(isocyanurate-urea) with a non-equilibrium, co-continuous morphology with a size scale of ≈ 100 A.

Structure-property relations in non-linear, segmented copolyureas formed by reaction injection moulding, RIM

SummarySegmented copolyureas have been formed by RIM using a MDI-based polyisocyanate (RMA400) and mixtures of a polyether triamine (Jeffamine T5000) and diethyltoluene diamine (DETDA) chain extender. Hard segment (HS) content was varied between 35 and 65% w/w at a constant overall stoichiometric ratio of -NCO to -NH2 groups of 1.03. All the copolyureas were translucent and DSC confirmed their totally amorphous structure.The copolyureas were shown by dynamic mechanical-thermal analysis to possess a two-phase morphology comprising polyether soft segments of constant Tgs of −40°C and aromatic polyurea hard segments with TgH increasing from 215 to 236°C as HS content increased. The ratio of flexural moduli at −35 and 65°C, decreased from 4.9 to 2.2 at 65% HS, and mechanical integrity was retained at temperatures in excess of 250°C, with flexural moduli of 10MPa at 270°C.Tensile stress-strain studies showed the polyureas to range from semi-rigid elastomers to stiff plastics with moduli greater than IGPa. Postcuring significantly improves materials toughness at high HS contents.