Jan 14 (Reuters) - Canada’s main stock index opened higher on Thursday, helped by strength in both energy and materials stocks, while optimism about a large U.S. stimulus and upbeat Chinese export data further boosted sentiment. * At 9:31 a.m. ET (1431 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was up 35.51 points, or 0.2%, at 17,970.25.