Which form of alginic acid, which is present in alginate, provides strength to the impression material?

Alpha-d-mannuronic acid provides flexibility, which is extracted from certain brown seaweed, is a linear copolymer of β-D-mannuronic acid and α-L-guluronic acid. Mannuronic and guluronic acids are epimers. It is known that the block structure within the alginic acid can vary significantly. Alginates with higher guluronic acid levels normally show a stronger interaction with calcium, and hence, yield greater gel strength. For impression-making purposes, the alginate is richer in mannuronic acid. Ref: Phillips 12 ed page no 171