We present proton-proton multiple-quantum investigations on a series of monomodal and strongly bimodal end-linked poly͑dimethylsiloxane͒ model networks. A robust pulse sequence characterized by a well-defined double-quantum Hamiltonian along with a specific normalization approach is used to obtain double-quantum build-up curves. These curves are analyzed in terms of the spin dynamics of a local subsystem of monomer-fixed spins, where analytical fitting functions yielding residual dipole-dipole coupling constants are derived on the basis of exact solutions provided by simulations. Further employing the novel experimental strategy of double-quantum preselection of elastically active network chains, it is shown that the network response is purely heterogeneous, and that the data can be analyzed in terms of distributions of local dynamic order parameters using different models. The results yield consistent proof that local chain order in bimodal networks obeys a linear mixing law of short-and long-chain components. The order parameter distribution in a long-chain monomodal network is found to be surprisingly narrow, with a rather high average order parameter. Implications on the validity of present theories used to explain order and dynamics in networks are discussed.

We present a study of the structure of Tetra-PEG model networks, using proton multiple-quantum NMR at low field in combination with computer simulations. Tetra-PEG is a novel high-performance hydrogel designed by combination of two symmetric tetra-arm macromonomers. In contrast to conventional hydrogels, which are highly heterogeneous due to fixed concentration fluctuations, Tetra-PEG exhibits a much less heterogeneous microstructure as indicated by previous light and small-angle neutron scattering studies. Here, the local-scale connectivity inhomogeneities, i.e., the sol and dangling polymer chains as well as the typical network connectivity defects resulting from the AÀB reaction of four-arm macromonomers, are quantified experimentally for the first time, studying as-prepared Tetra-PEG hydrogels, cross-linked at different polymer concentrations and stoichiometries. To this end, we developed a novel approach for the analysis of double-quantum buildup curves consisting of well-distinguishable components with different segmental dynamic order parameters, benefitting from the superb large-scale homogeneity of the samples. As a model for each component we suggest different connectivity modes between the macromonomers-mainly regular single links and double links between individual stars as well as other network defects with lower order parameters. To support the model, we report results from computer simulations with the bond-fluctuation model, which confirm the concentrationdependent trends of the network and double-link fractions.

Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl(2) as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ∼70%. For Ni(dppp)Cl(2)-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.

We present atomic force microscopy results and corresponding computer simulations on crystallization of quasi-2D monolayers of polyetheleneoxid adsorbed onto bare silicon wafers. Fingerlike branched patterns with a characteristic width w resulted from crystallization at temperatures T c below the melting point T m . w decreased exponentially as ͑T m 2 T c ͒ increased. The patterns are explained by a simple model considering the interplay of transport on the surface and the probability of attachment to the crystal. [S0031-9007(98)05902-X] PACS numbers: 61.41. + e, 81.10.Aj Crystallization can be considered as a process of molecular self-assembly, resulting in fascinating patterns such as snowflakes or spherulites [1][2][3][4][5]. The growth of polymeric crystals is complicated by the need of cooperative movements of the large number of connected monomers [6][7][8][9][10]. Problems such as chain folding [11,12], lamellar thickening [13,14], fractionation effects [6,8], or chain diffusion [15][16][17] affect molecular organization and consequently the resulting patterns. Specific molecular architectures [18,19] or restricted geometries [20 -22] impose further constraints. The influence of a solid surface may modify or even prevent polymer order [20][21][22]. Here, we focus on crystallization close to an interface in quasi-2D geometry [23] for strongly adsorbed polymers. Although there is an extensive literature [2,6-19] on the crystallization of polymers, we are still lacking comprehensive insight, especially for crystallization in thin films and close to interfaces. Recent work indicates that polymers may be unable to crystallize in ultrathin films [20]. Competition between adsorption and crystallization, difficulties to nucleate a crystal, and other surface induced effects may be anticipated.For our studies, we used low molecular weight polyethyleneoxid (PEO), a well investigated polymer [6,8,10,12,13,24], adsorbed onto the oxide surface of UV-ozone cleaned silicon wafers. Samples were prepared from dilute toluene solutions of dried [25] polymer by spin coating a thin film (about 50-100 nm) onto the wafers. Annealing these films in the molten state led to pseudodewetting [26] thereby forming holes and, eventually, tiny droplets on top of an adsorbed monolayer. Pseudodewetting could be arrested by crystallization which always started in the thick parts of the sample, e.g., at the rims, and not within the monolayer. This allowed the initial nucleation and the subsequent crystal growth process to separate in space. Consequently, our resulting patterns were due exclusively to crystal growth and were not affected or modified by nucleation processes within the monolayer [27].Atomic force microscopy (AFM) allowed visualization of the resulting patterns (see Fig. 1). The molecules, predominantly planar within the adsorbed layer [24], could crystallize by attaching to the already crystalline rim. They thereby attained a mostly vertical and highly stretched conformation (schematically indicated in the inset of Fig. 1). ...