مروری بر جنبه های ترمودینامیکی اسفنج های گرمانرم

نوع مقاله : مروری

نویسندگان

گروه مهندسی پلیمر، دانشکده مهندسی شیمی، دانشگاه تربیت مدرس، تهران، ایران

چکیده

در دو روش رایج برای تولید اسفنج‌های بسپاری، از عامل‌های اسفنج‌ساز شیمیایی و فیزیکی استفاده می‌شود. عامل اسفنج‌ساز فیزیکی در فشار و دمای بالاتر از شرایط بحرانی آن، تا حدی در بسپار حل شده و در اثر ایجاد افت فشار سریع یا افزایش دما در مخلوط، طی سه مرحله هسته‌گذاری، رشد و به ­هم‌ پیوستگی، ساختار اسفنج شکل می‌گیرد. در مرحله هسته‌گذاری، در اثر ایجاد فراسیر ‌شدگی، ناپایداری ترمودینامیکی در مخلوط‌ ایجاد شده که موجب تمایل مولکول‌های حلال برای انتقال فازی از حالت فوق‌بحرانی به حالت گاز در جهت کاهش ناپایداری می‌شود. با غلبه بر سدی از انرژی، انرژی آزاد سامانه کاهش یافته و هسته ­های پایداری از مولکول‌های گاز در حجم‌های آزاد بین زنجیرهای بسپارشکل می‌گیرند. با نفوذ مولکول‌های گاز به درون هسته‌ها، رشد و به ­هم پیوستگی آن­ ها رخ می ­دهد که سرانجام رشد متوقف شده و ساختار اسفنج تثبیت می‌شود. با افزایش بازده مرحله هسته‌گذاری، تعداد هسته‌های پایدار بیش ­تری ایجاد شده و میزان رشد و به­ هم‌ پیوستگی آنها نیز کاهش می‌یابد. بنابراین پیش‌بینی میزان سته‌گذاری عاملی مهم در کنترل ساختار اسفنج‌های گرمانرم بوده و بررسی مرحله هسته‌گذاری به کمک نظریه‌های هسته‌گذاری انجام‌پذیر است. نظریه کلاسیک هسته‌گذاری روش اصلی و اولیه بررسی پدیده هسته‌گذاری در اسفنج‌های گرمانرم بوده که به دلیل ناهمخوانی مناسب نتیجه­ های آن با نتیجه­ های آزمایشگاهی، اصلاحاتی بر روی آن آنجام شده است. همچنین نظریه‌های رقیب دیگری همچون نظریه‌های عاملیت چگالی و میدان خودسازگار نیز
به ­وجود آمده‌اند. هدف اصلی این مقاله مروری، بررسی جامع نظریه کلاسیک هسته‌گذاری  و اصلاحات آن و بررسی اجمالی دیگر نظریه‌ها در اسفنج‌های گرمانرم است.

کلیدواژه‌ها

موضوعات

مراجع

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