Considerações parciais – Eletrofiação de poliuretanas

Nesta etapa do trabalho SPUs semicristalinas e contendo baixo teor de PEG foram sintetizadas e processadas com sucesso pela técnica de eletrofiação. As condições de processamento relativas à solução polimérica (concentração e mistura de solventes) e parâmetros de operação (campo elétrico aplicado, distancia entre agulha e coletor e taxa de fluxo de solução) foram otimizados, permitindo a obtenção de fibras lisas, uniformes e sem formação de grânulos (beads). A influência destes parâmetros sobre a morfologia das fibras foi avaliado, permitindo a confecção de filmes eletrofiados uniformes. As propriedades térmicas e dinâmico-mecânicas dos filmes são dependentes de sua estrutura (filmes densos ou eletrofiados) bem como da composição das poliuretanas empregadas.

5. CONCLUSÕES

A síntese de poliuretanas segmentadas e não reticuladas foi realizada com sucesso graças aos cuidados adotados no controle de temperatura, da estequiometria dos reagentes e para evitar a contaminação com água. A variação da composição das SPUs, seja pela proporção entre os macrodióis ou pela escolha dos diisocianatos e extensores de cadeia, permitiu compreender as relações entre a composição, a estrutura e as propriedades, fundamentais para o

design de novos materiais.

A síntese de poliuretanas binárias e ternárias permite que as propriedades intrínsecas de cada macrodiol sejam combinadas. Os blocos (macrodióis) que constituem as SPUs são imiscíveis, o que resulta em SPUs heterogêneas. Entretanto, o bloco PTMC atua de forma a favorecer a miscibilidade parcial dos blocos PEG e PLLA, o que se reflete não apenas nas propriedades, mas também na morfologia das SPUs. Este papel do PTMC nas poliuretanas talvez seja a causa de que SPUs ternárias apresentam propriedades únicas, não encontradas em SPUs binárias. Devido à separação de fases, o PLLA influencia fortemente as propriedades das SPUs. A existência de fases ricas em PLLA resulta em filmes auto suportados em temperaturas próximas a 40 °C. O teor de blocos PEG determina a hidrofilicidade e capacidade de intumescimento em água das SPUs. Também determina como estas propriedades variam com a temperatura e, portanto, o caráter responsivo à temperatura.

A substituição do 2,4-TDI pelo HDI como diisocianato permite o melhor controle da composição das SPUs. Além disso, a simetria do HDI influencia o grau cristalinidade e a extensão da separação de fases, ocasionando alterações nas propriedades e na morfologia das SPUs. A substituição do extensor de cadeia BDO pelo DMPA resultou em SPUs ramificadas e com capacidade de intumescimento em solução aquosa dependente do pH (SPUs responsivas ao pH). A produção de membranas eletrofiadas a partir das SPUs foi possível pelo estabelecimento das condições adequadas de processamento. Em comparação a membranas densas, as membranas eletrofiadas apresentam elevada área superficial, baixa densidade e estrutura de poros interconectados.

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7. ANEXOS