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In this study , the molecular surface structures of two different types of QAS - incorporated PDMS systems were investigated in different chemical environments using sum frequency generation vibrational spectroscopy ( SFG ) .
 Chemical: PDMS
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: PDMS